In recent years, hydrofluorocarbons (HFC) that have zero ozone depletion potential (ODP) as well as a relatively low global warming potential (GWP) have been used as refrigerants for refrigerators. A difluoromethane refrigerant (HFC-32) has a low GWP that is about ⅓rd to ¼th of that of other refrigerants currently used (e.g., R-410A which is a mixture of difluoromethane and pentafluoroethane and R-407C which is a mixture of difluoromethane, pentafluoroethane and 1,1,1,2-tetrafluoroethane). Moreover, the difluoromethane refrigerant also has a coefficient of performance (COP) higher than that of R-410A, R-407C and the like by about 5 to 13% and therefore is a preferable refrigerant from the viewpoint of energy-saving (see Non-Patent Document 1).
Patent Document 1 discloses an ester of pentaerythritol and a fatty acid that is used as a refrigerant oil for the difluoromethane refrigerant. However, the ester disclosed in Patent Document 1 is not satisfactory in that it does not exhibit sufficient miscibility with the difluoromethane refrigerant, for example.
Patent Document 2 discloses a use of a mixed ester obtained by reacting pentaerythritol, i-nonanoic acid, and i-butyric anhydride in a molar ratio of 1:3:0.5 in the process of lubricating a ceramic-containing engine. However, Patent Document 2 does not disclose or suggest miscibility of the mixed ester with the difluoromethane refrigerant.